As authorities investigate a possible terrorist attack at a Christmas market in Berlin, experts say the public may see an increased law enforcement presence at holiday events in the U.S. in the days ahead but there is no need to panic here.

On Monday night, a truck rammed through a crowd at the market outside Kaiser Wilhelm Memorial Church. At least nine people were killed and dozens more were injured. Witnesses said it appeared to be a deliberate act, but police have not yet confirmed a motive.

"The United States condemns in the strongest terms what appears to have been a terrorist attack on a Christmas Market in Berlin, Germany, which has killed and wounded dozens... We also extend our heartfelt condolences to the people and Government of Germany. We have been in touch with German officials, and we stand ready to provide assistance as they recover from and investigate this horrific incident," National Security Council spokesman Ned Price said in a statement.

According to John Cohen, former counterterrorism coordinator for the Department of Homeland Security, there are several indicators that will lead German investigators to suspect an act of terrorism:

The tempo of attacks in Europe over the last 18 months

The fact that ISIS has specifically mentioned carrying out attacks in Europe during the Christmas season

Recent government warnings about possible attacks

A very tense and polarized environment in Germany right now

The incident occurred hours after a gunman killed the Russian ambassador to Turkey while shouting “Allahu Akbar! Do not forget Aleppo!” There was also a shooting at a mosque in Zurich earlier in the day that injured three. According to the Associated Press, police in Switzerland are not considering that shooting terrorism, but they say it is too early to tell if there is any link to the Berlin incident.

Anthony Roman, counterterrorism analyst and CEO of Roman & Associates, said all of these events may push agencies in the U.S. to step up their already heightened state of alert at a sensitive time.

“The intelligence agencies will be on high alert gathering as much info as is available both through computer communication, chatter on the waves of mobile phones, and through their sources on the ground, both in the Middle East and at the location of the incident,” Roman said.

The FBI will likely distribute updates to local law enforcement agencies across the U.S. about potential threats. Police departments will be protecting key strategic targets, including shopping malls and outdoor markets, with an increased presence both in public and behind the scenes.

“I can safely say that law enforcement and counterterror authorities are extraordinarily concerned about the potential for attacks during the holiday season,” said Cohen, now a professor at Rutgers University.

“It’s as high as it can get,” he said of the alert level.

Scott Stewart, vice president of tactical analysis at global intelligence firm Stratfor, said there have been several alerts for possible attacks on Christmas markets in Europe and some foiled plots, including an attempt by a 12-year-old to detonate a nail bomb at a market in another German city last month.

“Definitely the intent is there,” Stewart said. “The environment is more intense in Europe than the U.S., but that doesn’t mean there’s no threat here.”

Carrying out an attack days before Christmas would carry symbolic significance, Roman said, particularly if the suspect is motivated by religion. The timing of an incident around a holiday also seems to attract more media attention.

Cohen said the timing may be as much a matter of utility as anything else. Holiday markets draw large crowds of potential victims in close quarters.

“What these individuals are looking to do is cause as many casualties and fatalities as they can… The target environment is much richer,” he said.

He added that the police presence is typically heightened during the holiday season, as well, which can lead to a swift response.

Counterterrorism experts warn that intelligence capabilities developed since the Sept. 11, 2001 attacks were not designed for preventing low-tech assaults by self-radicalized individuals, which many ISIS-inspired attacks in the U.S. have been. Law enforcement has not fully caught up with the evolution of the threat.

“They’re looking for those connections, those communications, and in many cases they don’t exist,” Stewart said.

In light of what appears to have happened in Berlin, he urged the public to exhibit situational awareness at holiday events, but not to overreact.

“It’s really just an example of why we need to be on continued alert,” he said. “That doesn’t mean paranoia and fear.”

Roman said there is potential for other coordinated attacks or copycat attacks elsewhere over the next week, although the information available so far does not point to coordination.

“I think it’s always good to be at a higher state of alert anytime there is an overseas attack,” he said.

Driving vehicles into crowds is not a new method of terrorism, but it is one that ISIS has specifically promoted in a recent magazine. A similar attack at a Bastille Day celebration in Nice, France in July left 86 victims dead.

“It’s just an old tactic,” he said. “It’s not overly effective, but it’s simple and can be deadly.”

There are steps that can be taken to prevent such an attack. Some cities in Europe have set up physical barriers around markets. Spike strips can be put in place to force vehicles to stop. Removing trash cans from a public space or not allowing people to carry backpacks and large packages can also deter attacks.

Such measures may dissuade a terrorist from targeting one event, but it is possible they will just move on to another. In an open society, there are simply too many soft targets to prevent every attempted attack.

“It’s hard to stop them all,” Stewart said. “That’s just a fact of life.”